Because of tightening market constraints, it is becoming more economically attractive to process higher acid crude oils. But along with the increased acidity comes problems in transportation and refinement of the crude oil due to higher metal corrosivity caused by the acid. Thus, it is necessary to develop a quick, accurate and inexpensive method for determining the acid content of a hydrocarbon fuel, particularly of crude oil.
A common measure of acid content in crude oil is Total Acid Number or “TAN.” Predicting the TAN of a crude oil fraction is typically based on lab-scale distillation or pilot plant distillations. Unfortunately, these smaller-scale distillations have limited applicability to actual crude refining, because the sample is altered in the process of measurement. Heating a sample containing carboxylic or naphthenic acids leads to cracking, which lowers boiling point by reducing the average molecular weight of the fraction. Distillation temperatures also cause decarboxylation—that is, the loss of carboxylic acid functional groups—which lowers the acid number. The time and cost it takes to isolate the physical fractions also limits the usefulness of smaller-scale distillations.
Bachler et al. (“Simulated Distillation for Biofuel Analysis”) describes simulated distillation for characterization of biodiesel, especially because carboxylic acid functionalities can interfere with analysis by physical distillation. In particular, Bachler reports that shorter chain fatty acids, as can be found in coconut oil, significantly change the distillation characteristics of the fuel.
Qian et al. (“Measurement of Total Acid Number (TAN) and TAN Boiling Point Distribution in Petroleum Products by Electrospray Ionization Mass Spectroscopy”, Anal. Chem., 2008, 80(3): 849-855) and US20070037288 describe a method to measure TAN and TAN boiling point based on negative ion electrospray ionization mass spectrometry (ESI-MS), using stearic acid as an internal standard. However, the method is limited to molecular weights of ≦650 g/mol (C44) and requires specialized ESI-MS techniques and expensive equipment.
U.S. Pat. No. 7,618,824 describes a method for evaluating the acidity of an oil, wherein the acid functionality of the oil is isotopically enriched, for example with 13C, 18O or 2H, and then measured using isotope ratio mass spectroscopy. Further specialized and expensive equipment is required to use this method.
US20020086434 describes a method for measuring acid distribution in a crude oil using chlorine negative ion chemical ionization mass spectrometry. The application does not teach or suggest separating fractions by chromatography, SDA, or correlation between paraffin boiling points and TAN. Also, specialized and expensive equipment is required for this method, thus reducing its cost effectiveness.
In the publications above, acidity of a crude was measured without distilling and titrating each distillation fraction, but ESI-MS and/or isotopic enrichment was needed to calculate TAN. What is lacking is a method for determining boiling point distribution of acids and for calculating TAN that does not expose the sample to time-consuming or harsh chemical conditions, but uses established processing techniques.